The present invention relates to a self-emissive display device of active matrix type that uses, for instance, organic electroluminescence (EL) elements. More specifically, the present invention relates to a display device of active matrix type that allows supplying, to emissive elements, current having an appropriate brightness display gradation (tone of luminance) according to display data.
In image display devices that use organic EL materials or inorganic EL materials as electro-optic materials, the luminance of light emitted by the electro-optic material varies depending on the current with which pixels are written. EL display panels are self-emissive type panels having an emissive element at each pixel. EL display panels have various advantages vis-à-vis liquid crystal display panels, in that the former allow achieving, for instance, faster response speeds, smaller temperature dependence of the response speed, a wider gamut of reproducible colors, and higher visibility through a wide viewing angle and high emission efficiency, thanks to self emission, as well as a higher contrast.
Organic EL displays are driven according to a dot-matrix scheme, in the same way as liquid crystal displays. In organic EL displays, however, the brightness of each emissive element is controlled by the value of the current flowing therethrough, i.e. organic EL elements are current-controlled. Organic EL displays are hence significantly different from liquid crystal display, in which each cell is voltage-controlled. Dot-matrix driving can be fundamentally divided in active matrix driving, in which display data is written at a selection period and driving takes place thereafter based on the written values, and passive matrix driving, in which driving based on the display data is carried out only at the selection period. The basic circuits of active-matrix type organic EL display panels are well known.
FIG. 7 is a diagram illustrating an example of an equivalent circuit of one such pixel. The dotted line in the figure encloses a pixel circuit 10. The pixel circuit 10 comprises an EL element 11 that is an emissive element, a first transistor (driving transistor) 12, a second transistor (switching transistor) 13 and a capacitance (capacitor) 14. The emissive element 11 is an organic electroluminescence (EL) element.
The driver circuit that drives the pixel circuit 10 is not shown, but the configuration of the driver circuit is similar to that of driver circuits of liquid crystal display panels, in which a matrix is driven through output of signals that denote changes in the intensity of voltage corresponding to a video signal. Driving of organic EL display panels, however, is different from liquid crystal display in that, as pointed out above, organic EL elements are current-controlled, while liquid crystal displays are voltage-controlled.
In FIG. 7, the driver circuit applies a voltage signal, corresponding to a video signal, to a source signal line 15. With a gate signal line 16 (scan line) in a selected state, the transistor 13 is energized, whereupon the voltage signal applied to the source signal line 15 is written on the capacitor 14 and is held there. The gate potential of the transistor 12 is maintained stably by the capacitor 14 even when the gate signal line 16 (scan line) is in a non-selected state. The organic EL 11 continues emitting light at a brightness corresponding to the current determined by the written gate potential, unit the next writing.
Hereafter, the transistor 12 that supplies current to the EL element 11 illustrated in FIG. 7 will be referred to as driving transistor, and the transistor that operates as a switch for selecting an element in a matrix, such as the transistor 13 illustrated in FIG. 7, will be referred to as switching transistor.
The panels in organic EL display panels of active matrix type are built using transistors made up of low-temperature polysilicon or amorphous silicon. For various reasons, however, such transistors are difficult to form such that the transistors have a uniform characteristic, and non-negligible characteristic variation is a common occurrence. Such transistor characteristic variation, in particular variation in the characteristic of a driving transistor, precludes achieving uniform brightness in the organic EL element, even when the same driving transistor is driven in the same way. Variation in the characteristic of driving transistors in a same panel gives rise to display non-uniformity within the display.
FIG. 7 is a diagram illustrating the basic configuration of a voltage-programmed pixel circuit that drives a respective pixel. In voltage programming, a voltage signal such as a video signal denoted by voltage magnitude or voltage intensity changes is applied for instance to a data signal line, a source signal line or a pixel, whereupon the voltage signal is converted to a current signal by, for instance, the driving transistor of the pixel circuit, and the EL element is driven on the basis of the current signal.
Current programming refers to a configuration, circuit or driving method in which a current signal such as a video signal denoted by current magnitude or current intensity changes is applied for instance to a data signal line, a source signal line or a pixel, and a current signal substantially proportional to the applied current signal, or a current signal resulting from subjecting the applied current to a predetermined conversion processing, is directly or indirectly applied to the EL element.
In the pixel configuration illustrated in FIG. 7, the transistor 13 carries out a switching operation, as the name of switching transistor implies. Therefore, a variation in this transistor is comparatively non-influential to the overall characteristic. The transistor 12, called the driving transistor, however, drives the EL element by receiving the input of a video signal denoted by voltage intensity changes, and converting the video signal to a current signal. The driving transistor 12, therefore, carries out an analog operation, and hence any characteristic variation in the driving transistor 12 gives rise to variation in the converted current signal. The characteristic of the driving transistor 12 exhibits ordinarily a variation of 50% or higher.
In voltage programming, though, the charge-discharge ability of source signal lines and the like is high, both in low-gradation regions and high-gradation regions, and there occurs virtually no display non-uniformity caused by insufficient writing.
Display non-uniformity caused by the above-described transistor characteristic variation can be mitigated using a configuration based on current programming. Current programming, however, is problematic in that the driving current is small in low-gradation regions, which precludes achieving satisfactory driving on account of the parasitic capacitance of the source signal line 15.
In order to solve this problem, Japanese Patent Application Laid-open No. 2007-179037 discloses a method that combines the advantages of the above-described current programming and voltage programming. Also, Japanese Patent Application Laid-open No. 2006-301250 discloses the feature of measuring a threshold voltage (hereafter, an input voltage that does not contribute to gradation display will be referred to as threshold voltage) of the transistors that drive each EL element, and storing the measured threshold voltage for each EL element. The stored threshold value is used for generating a gradation execution voltage in accordance with display data, such that the generated gradation execution voltage is applied to the transistors that drive respective EL elements.
Threshold voltage can also be referred to as shift voltage, wherein voltage proportional to gradation data is shifted, in the correlation between the gate voltage of the driving transistor and the luminance of emitted light, to set a linear relationship between luminance of emitted light with respect to gradation data.
The above-described method, however, cannot completely compensate for the initial variation of the electron mobility and of the threshold voltage (hereafter, Vth) of the transistor characteristic, or for the fluctuation of the foregoing over time. FIG. 8 illustrates schematically the fluctuation over time of the above two characteristics in an example of a transistor made up of amorphous silicon. In this transistor, Vth rises in the figure from Vthi to Vthn, and electron mobility drops from αi to αn in the figure, on account of internal deterioration as driving hours go by. Therefore, when Vdata, which is the gradation signal, is constant, the driving current drops from Idi to Idn, and brightness drops accordingly in proportion to the drop in driving current. The characteristic change in such a driving transistor varies depending on the individual transistor in the matrix. Therefore, display brightness non-uniformity occurs in the display surface as time goes by, even when countermeasures are taken to cancel initial non-uniformity of display brightness. Initial variation can be linked to the occurrence of initial non-uniformity of display brightness by replacing the characteristic that exhibits fluctuation over time in FIG. 8 by the initial characteristic of each transistor.
In CMOS there holds the relationship μ=2LIds/WCi(Vg−Vth)2 between the above-described electron mobility (μ) and other characteristics. In the above expression, L is the channel length, Ids is the drain current value in the saturation region, W is the channel width, Ci is the capacitance per unit area of the gate insulating layer, Vg is the gate voltage and Vth is the threshold voltage. It becomes apparent therefore that the fluctuation in electron mobility exerts a significant influence on the transistor characteristic, in particular on the ratio of node current change relative to gate voltage change.